Star cluster's triple baby boom puzzles astronomers

Astronomers are puzzling over a strange, ancient star cluster that hosts three generations of stars instead of the normal one. Some researchers say it might be the remains of a small galaxy that was dismembered by the Milky Way; if so, it could help clarify the murky picture of our galaxy's beginnings billions of years ago.

Globular star clusters are dense groupings of hundreds of thousands or millions of stars that dot our Milky Way galaxy. They contain the galaxy's most ancient stars, which date back to perhaps just a few hundred million years after the big bang 13.7 billion years ago.

Each cluster was thought to have formed in a single burst of star formation, since that single baby boom should have blown away the gas needed to form future generations of stars.

Now, a Hubble Space Telescope survey of one cluster that contains three distinct generations of stars is challenging that long-held notion.

Three populations

Giampaolo Piotto of the University of Padua in Italy led the group that analysed the stars in a globular cluster called NGC 2808, which is about 30,000 light years from Earth. Watch an animation showing what it might look like to fly into the heart of the cluster.

The brightness and colour of the stars indicate there are three populations that seem to have formed in different episodes, perhaps separated by a few tens of millions of years each.

"NGC 2808 was just considered a normal globular cluster and no one was expecting this ability to see three distinct stellar populations," Piotto told New Scientist. "This result says globular cluster stars are not as simple as we are teaching to our students."

Dismembered galaxy

Volker Bromm of the University of Texas in Austin, US, who is not a member of the team, says the cluster's unexpectedly complex star formation history means it is probably the remains of a dwarf galaxy that had its outer stars ripped away by the Milky Way's gravity, leaving only a dense core behind. This would make it similar to the galaxy's most massive globular cluster, Omega Centauri.

"Omega Centauri is sort of a ghost of a former dwarf galaxy from the early universe," Bromm told New Scientist. "It seems that this new system definitely also was part of a dwarf galaxy - it clearly fits, I think, in that class."

Further study of such clusters could shed light on what was happening in the very early universe, only a few hundred million years after the big bang, he says: "One can think about these globular clusters as fossils encoding some of this early star formation history."

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NGC 2808, one of the galaxy's most massive star clusters, appears in this image by the Hubble Space Telescope. It may be all that is left of what was once a small galaxy (Image: Hubble/ESA/NASA/G Piotto/U Padua/A Sarajedini/U Florida)